1. Field of the Invention
The present invention relates to video signal receivers which receive high definition television (HDTV) signals and, in particular, to employing a trellis decoder to decode a received VSB-modulated HDTV signal after it has been demodulated and comb-filtered to reject NTSC co-channel interference.
2. Background
In data transmission systems, data, such as audio and video television (TV) data, is transmitted to a plurality of receivers. In the field of television signal transmission systems, the current NTSC (National Television Systems Committee) standard of transmission is being replaced by a higher-quality system, known as HDTV, or the ATSC-HDTV standard (see United States Advanced Television Systems Committee, ATSC Digital Television Standard, Document A/53, Sep. 16, 1995). Such HDTV signals are of the VSB-modulated (Vestigial SideBand) type proposed by the Grand Alliance in the United States.
The ATSC-HDTV standard requires an 8-VSB modulated transmission system which includes forward error correction (FEC) as a means of improving system performance. Referring now to FIG. 1, there is shown a simplified block diagram of the forward error correcting (FEC) aspects of a HDTV transmission system 100. System 100 contains a Reed-Solomon encoder 103, followed by a byte interleaver 104, and a trellis encoder 105 on the transmitter side 101. At the receiver end 121 there is a corresponding trellis decoder 125, byte deinterleaver 124, and Reed-Solomon decoder 123.
In such a system, data signals are first encoded in accordance with a given code or encoding scheme, such as a convolutional, or trellis, code, by trellis encoder 105. The trellis code employed is a rate 2/3 TCM (trellis coded modulation) code, as described in the ATSC Digital Television Standard. This code is implemented by coding one bit using a rate 1/xe2x96xa1, 4-state convolutional encoder, and adding an FEC uncoded bit, which is differentially precoded. Each set of three encoder output bits is then mapped to an 8-VSB modulator symbol by modulator 106, and transmitted over a given communications channel and transmission medium 150. For example, the modulated, encoded HDTV signal may be transmitted as a terrestrial RF signal through the air. The transmitted signal contains digital data representing HDTV image and other information in the form of multilevel symbols formatted into groups of successive fields, each field comprising a field segment, a plurality of data segments, and associated sync components.
The HDTV receiver 121 receives the transmitted signals. Demodulator 126 is used to demodulate the signal to provide a demodulated baseband signal; and trellis decoder 125 is used to decode the demodulated signal to obtain the original data.
Due to the fact that NTSC and HDTV signals will coexist in the terrestrial broadcast channel for a number of years, it is important for the receiver 121 to reject possible NTSC co-channel interference. The elimination of NTSC interference may be performed by an NTSC rejection filter, such as a comb filter, added to the demodulator. The comb filter is typically a 12-symbol one-tapped delay line with signal attenuating nulls at or near the NTSC carriers.
Thus, when the HDTV receiver detects NTSC co-channel interference, it filters the demodulated signal to remove the NTSC co-channel interference that would otherwise arise, before performing trellis decoding. In the non-NTSC interference case, to avoid unnecessary filtering and undesirable effects of such filtering, the comb filter is not applied.
When no NTSC interference is detected, the optimal trellis decoder for the AWGN (Additive White Gaussian Noise) channel is a 4-state Viterbi decoder with the Euclidean metric. See G. Ungerboeck, Channel Coding with Multilevel/Phase Signals, IEEE Trans. Inform. Theory, vol. IT-28, pp. 55-67, January 1982. Thus, in performing the decoding, the trellis decoder 125 typically employs an Euclidean metric, which can provide optimal decoding when there is no NTSC interference. However, when NTSC interference is present, the use of the NTSC rejection (comb) filter introduces correlation in the noise (Additive Colored Gaussian Noise), such that the optimal trellis decoder is much more complex. Therefore, an optimal trellis decoder is typically used where there is no NTSC co-channel interference, and a partial response trellis decoder is used whenever the NTSC rejection filtering is employed.
Such systems employ 12 intra-segment interleaving (deinterleaving) in the trellis encoding (decoding), in which 12 identical encoders (decoders) are used. This permits implementing the trellis decoder 202 in the NTSC interference case as an 8-state (partial response) trellis decoder, and as a 4-state (optimal) trellis decoder 203 in the non-NTSC interference case. By employing the 12 encoder/decoder interleaving, each of the identical decoders of the trellis decoder for the NTSC interference case views the comb filter with a 1-symbol delay (instead of 12). The advantage of this architecture is that the optimal trellis decoder can be implemented with an 8-state Viterbi decoder. See United States Advanced Television Systems Committee, Guide to the Use of the ATSC Digital Television Standard, Document A/54, Oct. 4, 1995.
Referring now to FIG. 2, there is shown a block diagram illustrating the HDTV trellis decoding performed by receiver 121 of system 100 of FIG. 1, with and without NTSC interference, for each of 12 sequential decoders of trellis decoder 125. Symbol-level signal data is received from demodulator 126 (FIG. 1). In a first (NTSC interference) path, the demodulated signal is filtered by NTSC rejection (comb) filter 201, and this filtered, demodulated signal is decoded by partial response 8-state trellis decoder 202. The 8-state decoder 202 receives at its input a partial-response signal plus noise, because it is comb-filtered. This partial-response signal, which is derived from 8-VSB symbols, is also known as 15-VSB since it has 15 amplitude levels. In a second (non-NTSC interference) path, the demodulated signal is not filtered, and is decoded by optimal 4-state trellis decoder 203. Switch 204 selects the appropriate decoded signal depending on whether NTSC interference is detected.
As will be appreciated, there may be only a single trellis decoder 125 which is used to implement both 8-state trellis decoder 202 and 4-state trellis decoder 203, depending on whether demodulator 126 detects NTSC interference or not. Or, trellis decoder 125 may include separate decoders 202, 203, one of which is selected depending on whether NTSC interference is detected. Further, comb filter 201 is included in demodulator 126. It is selected, or applied, by demodulator 126 when it detects NTSC interference. Thus, when demodulator 126 detects NTSC interference, it outputs a comb filtered, demodulated signal to decoder 125, and also instructs decoder 125 that NTSC interference has been detected so that decoder 125 can use the 8-state trellis decoder 202. Conversely, when demodulator 126 does not detect NTSC interference, it outputs a non-comb filtered, demodulated signal to decoder 125, and does not instruct decoder 125 that NTSC interference has been detected, so that decoder 125 can use the 4-state trellis decoder 203. This functionality is illustrated in the process flow of FIG. 2.
Both the optimal 4-state trellis decoder 203, and partial response 8-state trellis decoder 202, employ the Euclidean metric or some variation of it in current implementations. In the non-NTSC interference case of decoder 204, this results in an optimal trellis decoder and optimal results, as described above. However, the 8-state trellis decoder 202 with the Euclidean metric has a performance degradation of about 3.0 to 3.75 dB with respect to the non-NTSC interference case. A suboptimal truncated non-Euclidean trellis decoder metric with better performance than the Euclidean metric is described in a copending U.S. patent application Ser. No. 09/603,655 filed Jun. 26, 2000 for Markman et al. Particularly, its two simplest implementations give 1.0 and 1.5 dB improvement over the Euclidean metric.
A signal processing method according to the present invention is included in a system for receiving a trellis coded signal containing digital data. A received signal is demodulated to produce a signal that can be trellis decoded using a Euclidean metric. The demodulated signal is subjected to filtering which produces an output with correlated noise. The filtered signal is trellis decoded using a recursive non-Euclidean metric.